In order to get him to 5th best, your minimum plate appearances would have to be 90. In order to claim he had the 40th-best OPS, the minimum plate appearances would be 140.
Would you say that Austin Hays led the Orioles in OPS last year?
One player Mountcastle's age or younger had a higher OPS in the IL last year with 200+ plate appearances, Luis Robert (top 3 overall prospect). Austin Riley (top 40 prospect going into 2019) topped him over 194 IL plate appearances.
Isn't that backwards? If Mountcastle were one of the top prospects in baseball he would have many MLB at bats by 23 or 24. One of the reasons Elias could leave him in AAA all year last year is that he wasn't running roughshod over the league, yes he's pretty young but he's a 1B/LF/DH with the 40th-best OPS in the IL, 4th or 5th-best on the Tides.